1. Field of the Invention
This invention relates to a transponder/cathodic protection system that generates surface detectable signals that can be used to monitor and measure the effectiveness of a buried cathodic protection circuit for buried metal objects, in particular, buried metal pipes. The transponder, which is buried, converts the voltage and amperage generated by a typical cathodic protection circuit into radio frequency waves that can be detected remotely above ground by any suitable transmitter-receiver tuned to the transponder frequency.
2. Description of Prior Art
Underground metal objects, such as natural gas distribution and transmission pipelines, tend to build up electrical charges caused by the earth's magnetic field. The metal object also serves as a conductor between soils of differing chemical composition and conductivity, in effect, forming a battery and setting up circulating currents in the pipe-soil system. After a length of time, depending upon soil conductivity and conditions, the metal object will become sufficiently charged such that an electrical discharge will occur from the metal object to the ground, causing corrosion of the metal object. In the case of buried metal pipes, the electrical discharge causes metal particles to be carried away from the pipe, thereby pitting the pipe which, in turn, can cause a hole to develop at the point of discharge. Various devices have been used to counteract this electrolytic process, the most common of which is an electrical power supply, more commonly called a cathodic protection rectifier, which produces a rectified low voltage DC current. The DC current output of the rectifier is then connected to the metal object to effectively counteract or negate the electrical charge build up in a manner which prevents damage to the metal object. The metal object is, thus, made cathodic so that plating action occurs at the metal object and deplating occurs at a buried sacrificial anode.
U.S. Pat. No. 5,216,370 teaches a cathodic protection monitoring system which provides IR drop free cathodic protection potential measurements which are indicative of the effectiveness of the cathodic protection system. The system measures the polarized potential between a reference electrode and a coupon subsequent to decoupling the coupon from the protected structure. The system controls the time or times at which the potential is measured in order to ensure that the potential is measured only after the polarized potential has achieved a relatively steady state value. The system includes an above ground test module including a timing circuit and voltmeter, the test module being removably electrically attached to a switch network by way of terminals. The reference electrode is electrically coupled to the test module by way of an electrical cable; similarly, the coupon and metal structure are electrically coupled to the switch network using electrical cables.
A cathodic protection measurement system including a coupon buried near and electrically connected to a pipeline so as to receive the same level of cathodic protection current as the pipeline is taught by U.S. Pat. No. 5,469,048. A test wire is connected to the pipeline and routed to a normally closed contact switch located at an access point of an above ground test station. The switch is also connected to a coupon wire, which is routed and connected to the coupon to complete the electrical connection. A reference electrode having a measuring surface contacting the soil close to the buried coupon includes an electrode wire provided to the access point, the access point comprising a tubular access tube which penetrates the soil and extends to the pipeline being protected. A voltmeter is connected between the switch and the reference electrode wire, and the switch is then opened to electrically isolate the coupon from the pipeline. U.S. Pat. No. 5,144,247 teaches an apparatus for measuring cathodic protection voltage levels on a concealed conductive structure which includes a probe having a standard half cell reference electrode, a working electrode, and an auxiliary electrode mounted in a fixed spacial relationship to each other, wherein the reference electrode, working electrode and auxiliary electrode are each in contact with an electrolytic solution. A voltage measuring device is provided for receiving the voltage from the reference electrode. A switch is included for connecting and disconnecting the working electrode to the common reference point. See also U.S. Pat. No. 5,446,369 which also teaches a corrosion monitoring system.
Conventional cathodic protection surveying in an urban environment is a complicated and expensive process. Paving often renders it impossible to use a portable reference electrode. Buried reference electrodes can be lost as landmarks and paving shift over time. Monitoring the state of the cathodic protection of buried metallic gas distribution and transmission systems is an operation that is carried out by all system operators. The most widely accepted method of demonstrating adequate protection is to measure the pipe to soil potential with respect to a reference electrode. In practice, the reference electrode is placed in contact with the soil and a lead wire is attached to the main. The voltage between these two points is measured with a high impedance voltmeter. If the main voltage is correct with respect to the particular type of reference electrode, for example 850 millivolts for copper/copper-sulfate, the main is adequately protected. This measurement can be taken by either steady state or instant-off methods.
The urban environment offers particular challenges to carrying out the operation of cathodic protection monitoring. Often, a portable reference electrode can not be used when there is no unpaved soil nearby in which to insert the electrode. Measurements can be taken using a permanently buried electrode if lead wires are brought to the surface from the pipe and the electrode. One practice is to bring both of these lead wires to the surface in a common valve box that can be opened when a cathodic protection survey is being performed. These valve boxes can be buried, paved over, or otherwise "lost" in the shifting urban landscape.
In the case of pipelines located in remote areas, where physical connection to the cathodic protection system may not be possible and most certainly is not desirable, various devices have been used for visual observation from airplanes that routinely patrol the pipeline right-of-way to make a determination as to whether or not the systems are operating properly. The most common device utilized is simply a red light disposed on top of a pole that burns if the system is functioning properly and goes out if the system fails. Similar systems utilizing spinning pinwheels or the like which are motor driven from the output of the cathodic protection circuit are also known. Thus, when the circuit fails, the pinwheel will no longer spin. U.S. Pat. No. 3,860,912 teaches a power supply monitoring device for monitoring remote power line connected cathodic protection rectifier operation along a pipeline from pipeline patrol vehicles. The device comprises transmitter means disposed adjacent to each power supply rectifier and operably connected to the power line for transmitting a signal, monitoring means operably connected to the output of the cathodic protection rectifier for sensing the output current thereof, signal modulating means operably connected between the monitor means and the transmitter means for modulating the transmitter signal in accordance with the rectifier current level, receiver means disposed within the patrol vehicle for receiving the output of the transmitter means when the patrol vehicle is in the near proximity of the rectifier, and audio output means operably connected to the receiver means for indicating the cathodic protection rectifier current level. As in the case of other known devices, a significant portion of the monitoring device is disposed above ground and is physically connected to the below ground components.